Recent battery technology, such as is disclosed in U.S. Pat. No. 3,476,602 discloses the use of a molten alkali metal anode/anolyte and a molten sulfur/alkali metal sulfide catholyte separated by an alkali metal ion-permeable membrane/electrolyte. A cathodic current collector, or "cathode" is immersed in the catholyte. When the anode and cathode are connected through an external electrical circuit, electrons are discharged to this circuit from the anode with formation of positively charged alkali metal ions. These ions migrate through the membrane into the catholyte. Negatively charged sulfide (polysulfide) ions are formed in the catholyte by interaction of sulfur at the cathode surface with electrons received from the external circuit.
In one embodiment of such a battery, the membrane is in the form of a multiplicity of sodium-filled hollow glass fibers closed at one end and open at the other. The open ends of the fibers communicate with a reservoir of molten sodium and the fibers are immersed in the molten polysulfide catholyte, the anolyte (sodium) and catholyte being separated bya "tube sheet" through which the fibers pass in sealing arrangement. In this embodiment, a large anode area by a afforded by closely spacing a large number of the fine hollow fibers in a given cell or battery volume.
U.S. Pat. No. 3,791,868 discloses a battery cell of the preceding type in which the cathodic electrode is a metallic sheet wrapped into the shape of a coil and the alkali-metal filled fibers are disposed between successive wraps of the coil. The sulfur/sulfide (polysulfide) catholyte fills the spaces between adjacent fibers and between the fibers and foil wraps.
Contemplated operating temperatures for alkali metal/sulfur batteries range from about 200.degree.-400.degree. C. It is generally convenient, or even necessary, to introduce the sulfur to such a battery in the form of a melt, which may consist of sulfur as such or as a polysulfide of the alkali metal to be employed as the anolyte. Given the presence of the alkali metal in the cell, the latter can then be heated to the desired operating temperature and operated. If the cell is loaded with the sulfur-containing catholyte (and the alkali metal) just prior to being used, there is no reason to let the temperature drop so low that solidification of the catholyte will result. On the other hand, if it is not intended to operate the cell at the location where it is filled, or if operation is to be suspended, it is an obvious desideratum to be able to let the cell cool to ordinary temperatures. However, membrane breakage and consequent electrical shorting may result when the catholyte is allowed to solidify in contact with the membrane. Membranes in the form of fine hollow fibers are very thin and the fibers are usually surrounded by the catholyte. Consequently, membranes of this form are particularly susceptible to such damage.
Accordingly, it is an object of the present invention to provide an alkali metal/sulfur cell (or battery) in which the electrolyte-separator is a glass or ceramic membrane and which is so designed that the catholyte it contains can be removed from substantial contact with the membrane and then allowed to solidify.
A particular object is to provide a cell of the foregoing type in which the membrane takes the form of a plurality of hollow fibers or tubules.
A further object is to provide such a cell in which the solidified catholyte can be remelted and then redisposed in contact with the membrane. A corollary to the latter object is to be able to initially load the catnolyte (or the sulfur component thereof) into the cell as powdered (fluidized) solid which can subsequently be melted and relocated in contact with the membrane.
An additional object is to provide a method of shutting down operation of an alkali metal/sulfur cell whereby catholyte solidification will not result in membrane damage and the cell is not rendered incapable of being put back into operation.
Still other objects will be apparent from the following disclosure.